The current dealer already has had it for a very long time. obviously read for codes, found nothing and so unable to diagnose/find the fault. As Bob, (G'day Bob) suggested, another service centre might also do similar testing, takes all of 1 minute to do, and as a result of no codes, simply starts thinking of alternatives faults. Probably find it in a short time.
My son had an accident in a VW Golf Wagon, he rear ended someone, his fault. After repair the aircon would not work. Had it repairers who used two different autoelec folk over three days. No joy. Dealer of vehicle became involved and they couldn't fix it. Loss of vehicle for 5 days total. He drove it to me and I thought, front hit- repair, no aircon. what wiring was involved. Lifted bonnet. found wiring around RHS subframe behind headlight. Unwrapped the hard turn of loom sheath and found 1 yellow wire broken. Held it together and the aircon worked. All professional auto elecs and dealer mechanics over 5 days = nothing. Took 30 seconds to find. That is what you are up against sometimes. Think outside the square.